1. Field of the Invention
This invention relates to measuring spectral responses of optical elements. More particularly, this invention relates to improving spectral response measurements by mitigating polarization dependence.
2. Description of the Related Art
Most optical components produce a spectral response that depends on the polarization of incident light. This is often manifested as a polarization-dependent shift in the wavelength of a spectral characteristic (e.g. a reflection peak wavelength) of an optical component. If the incident light is highly polarized, this polarization dependence can cause significant shifts in the wavelength of the spectral characteristic. In many systems, the polarization of the incident light is varying and unknown. This results in unpredictably varying shifts in the wavelengths of the spectral characteristic.
One of the many optical elements having polarization-dependent shifts in wavelength is the fiber Bragg grating (FBG) element. A fiber Bragg grating (FBG) element is an optical element that is usually formed by photo-induced periodic modulation of the refractive index of an optical fiber's core. An FBG element is highly reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Other wavelengths are passed without reflection. The Bragg wavelength itself is dependent on physical parameters, such as temperature and strain, that impact on the refractive index. Therefore, FBG elements can be used as sensors to measure such parameters. After proper calibration, the Bragg wavelength is an absolute measure of the physical parameters.
FBG sensors typically include a tunable laser that interrogates an FBG element by sweeping across an optical spectrum that includes that Bragg wavelength. Alternatively, a broadband light source/tunable filter combination can be used. The sweeping light generates reflections from the FBG element that produce a spectral response of intensity verses wavelength. Since the spectral response peaks correspond to the Bragg wavelengths of the FBG element, by determining the changes in response peaks produced by the physical parameter of interest that parameter can be measured.
Unfortunately, in FBG sensors, the polarization-dependent wavelength shift can limit the achievable measurement accuracy and resolution. This is because the spectral response peaks change not only because of the physical parameter of interest, but also because of polarization induced wavelength shifts.
That polarization-dependent wavelength shifts can impact measurements is known, see reference Vines, Lasse, “Polarization Dependence in CHESS Fiber Optic Strain Monitoring System Based on Fiber Bragg Gratings”, Norwegian Defense Research Establishment doc #: FFI/RAPPORT-2002/03348, ISBN-82-464-0645-0. That reference describes attempts at mitigating the polarization problem by depolarizing the source radiation. However, when using narrow-bandwidth sources this puts stringent requirements on the depolarizer design, often beyond what is practically achievable.
Therefore, a new method and apparatus of compensating for polarization-dependent wavelength shifts would be beneficial.